Anodising of aluminium and its alloys

ABSTRACT

THIS INVENTION RELATES TO A METHOD OF ANODISING ALUMINIUM AND ITS ALLOYS, FOR EXAMPLE, N THE FORM OF WROUGHT OR CAST ARTICLES IN A SULPHURIC ACID ELECTROLYTE TO WHICH HAS BEEN ADDED EITHER DURING OR BEFORE ANODISING A SUBSTANCE WHICH WILL IMPART NITRATE IONS TO THE ELECTRLYTE. SUCH SUBSTANCES INCLUDE NITRIC ACID AND SOLUBLE NITRATES AND NITRITES.

United States Patent O US. Cl. 204-58 3 Claims ABSTRACT OF THEDISCLOSURE This invention relates to a method of anodising aluminium andits alloys, for example, in the form of wrought or cast articles in asulphuric acid electrolyte to which has been added either during orbefore anodising a substance which will impart nitrate ions to theelectrolyte. Such substances include nitric acid and soluble nitratesand nitrites.

CROSS-REFERENCES TO RELATED APPLICATIONS This application is acontinuation-in-part of an application for Letters Patent Ser. No.598,966 filed on Dec. 5, 1966 by J. M. Kape et al.

BACKGROUND OF THE INVENTION The anodising of aluminium and its alloys inelectrolytes including sulphuric acid solutions, thus producing sulphateions in the solution, is well known. The concentration of sulphuric acidemployed ranges from 92 gm. per litre up to over 370 gm. per litre andthe anodic coatings produced range in thickness from 2 microns or lessup to over 25 microns, the current densities ranging from 10 to 60amperes per square foot and the applied voltage varying between 12 and20 volts. The temperature f the electrolyte must be maintained atbetween 15 C. and 25 C. for optimum results.

Very hard anodic coatings have also been produced on aluminium alloys attemperatures below C., usually at 0 C., but the voltages required toproduce the current densities of to 60 amperes per square foot arehigher than those just referred to and may rise to as high as 100 voltsfor the production of hard coatings of a thickness of over 25 microns onsome alloys. The higher the voltage required to produce a given currentdensity the greater the amount of cooling needed to maintain theelectrolyte at the required temperature.

In addition some aluminium alloys, particularly those containing morethan 1% of copper, nickel or silicon or admixtures of these elements,are very diflicult to anodise or hard anodise in a sulphuric acidelectrolyte using conventional techniques to produce an anodic coatingof sufficient thickness to have a commercial value. Such alloys include,for example, those known under the nomenclature of British StandardsSpecifications 1470 and 1490 as LM6, H15, RR250 and LM4.

It is an object of the present invention to provide an improved methodof anodising aluminium and its alloys whereby the voltage required toproduce a given current density is lowered and the voltage rise duringanodising 3,562,121 Patented Feb. 9, 1971 "ice is reduced, in relationto those of normal processes, yet with the production of anodic coatingshaving properties equivalent to those of the coatings produced in thesenormal processes and with considerable savings in electrical energy andin the amount of cooling required to maintain the required temperatureof the electrolyte. A further object of the invention is to enableacceptable anodic coatings to be formed upon aluminium alloys hithertofound difficult to anodise.

SUMMARY OF THE INVENTION According to the invention there is provided amethod of anodising aluminium and its alloys in an electrolytecontaining a sulphuric acid solution comprising adding to theelectrolyte before or during the anodising operation a soluble compoundto produce nitrate ions in the electrolyte, the amount of the compoundbeing such as to enable an anodised surface to be formed on aluminium orits alloy.

Preferably, the amount of nitrate ion forming compound does not exceedthe equivalent of 45 gm. per litre of nitric acid in water.

The compounds used can be any water soluble compound containing nitrateor nitrite groups which, when in solution, will provide the requisitenitrate ions. Examples of such compounds are: ammonium nitrate, cupricnitrate, urea nitrate, guanidine nitrate, m. nitro-benzoic acid, m. orp. nitrophenol, aniline nitrate.

The electrolyte may be an aqueous solution containing 56 to 560 gm. perlitre of sulphuric acid and preferably is an aqueous solution containing140 gm. per litre of sulphuric acid.

The anodising operation may be carried out at an applied voltage between10 and 60. The current density used may be between 60 and 120 amperesper square foot.

The electrolyte temperature may be between 0 C. and 25 C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Some examples of the way inwhich this invention may be carried into practice will now be described.The initial examples are of the use of nitric acid as the ion producingcompound, since this is the most basic compound providing the nitrateions. Thereafter will be described the use of other compounds.

Current density in amperes per square foot Sulphuric Sulphuricacid-niacid electric acid trolyte electrolyte It should be noted thatthe voltage-current density relationship varies considerably with thealuminium alloy used.

T'wo samples of each of a number of different wrought aluminium alloys(identified below by the nomenclature of British Standards 1470 and1490) were separately anodised in the two baths referred to above at thesame voltage and temperature, a sample of a given alloy being anodisedat a given voltage and temperature for 1 hour in the sulphuric acidelectrolyte and an exactly similar sample being anodised for half anhour (at the same voltage and temperature) in the sulphuric acid-nitricacid electrolyte. The thicknesses of the anodic coatings were thenmeasured and are tabulated here:

Film thickness (microns) 140 gm./ litre 140 141128047 gm. .2 gm. Alloy(Brit1sh Stand- ,Tempcralitre litre ards nomenclature) Voltage ture, C.H2804 +HNOa It can be appreciated from these results that the additionof nitric acid to the electrolyte resulted in a much higher currentdensity for the same applied voltage, with consequently a more rapidgrowth of the anodic coating.

A sample of the alloy HE9 was anodised at a current density of 60amperes per square foot in an electrolyte consisting of a 140 gm. perlitre solution of sulphuric Film thickness (microns) SulphuricConvenacid-ni- Alloy (British Standards tional trie aeid nomenclature)electrolyte electrolyte The coating formed on the alloy HE with theelectrolyte according to this invention, showed a substantialimprovement in abrasion resistance over the hard anodic coating producedon this alloy by the conventional meth od. It will also be noted thatthis table includes the alloy LM6, referred to above and that theelectrolyte according to the invention enables an anodic coating ofcommercially acceptable thickness to be obtained.

Although in the examples given above the electrolyte had a sulphuricacid concentration of 140 gm. per litre it has been found that this mayvary between 56 and 5 60 gm. per litre.

Further examples of the use of nitric acid in various quantities forhard anodising aluminium alloys known as H15 and H30 are given in thefollowing table:

Current Anod- Film Temper- Voltdensity ising thickature, age (amps/ timeness Electrolyte 0. range sq.ft.) (min.) (mil) Alloy 280 gmjlitre H 80plus 14 gm./litre HNO3 3 20-90 48 28 2. 3 H80 3 -60 48 60 3. 0 H15 122-55 48 60 4. 0 H15 280 gm./litrc H SOl plus 2.1 gm./litre HNO3 1 20-9048 37 2. 4. H 1 20-50 48 3.0 H15 280 g1n./litre H280 plus 28 gm./litreHNO; 1 19-60 48 25 2. 2 H30 -3 22-53 48 60 4. 2 H15 280 grnJlitre H SO;plus 35 gm./litre HNOa 0 18-90 48 31 2.0 H30 0 21-45 48 60 3. 8 H15 280gm./litre H 80 plus 42 gin/litre HNO3 2 17. 590 48 33 2. 4 H30 2 18-5548 60 4. 2 H15 280 gm./litre H2804 plus 49 gnrllitre HNO;

acid maintained at 20 C. After one hour the voltage had risen from 20 toover 50 volts and the average thickness of the anodic coating producedwas 75 microns.

An exactly similar sample of the alloy HE9 was anoldised at the samecurrent density and temperature for one hour in an aqueous electrolytecontaining 140 gm. per litre of H 80 and 14.2 gm. per litre of HNO(specific gravity 1.42). The voltage only rose from 2 0 to 22 volts andthe average thickness of the anodic coating produced was 100 microns.

Two samples of the alloy HE9 were separately but similarly anodised toproduce anodic coatings 25 microns thick in respective electrolytes, onecontaining 140 gm. per litre of sulphuric acid and the other containing14.2 gm. per litre of nitric acid. The specific abrasion resistance ofboth the coatings, when measured at an air flow rate of 70 litres perminute by method 0 of the British Standard 1615: 196 1, was 2.0 grammesper micron of film thickness.

The sulphuric acid-nitric acid electrolyte is particularly useful forproducing thick hard anodic coatings. In the following table are set outthe film thicknesses ob- Any soluble compound which will produce nitrateions in solution (i.e. soluble compounds containing nitrate or nitritegroups) may be substituted for the nitric acid. The following tablegives the equivalent amounts of a number of such compounds in relationto 14 gm. per litre of nitric acid in order to give the same results:

The following table sets out experimental results obtained usingtheabove compounds for hard anodising aluminium alloys known under theBritish Standards Nomenclature H.15 and H30. The first example in thetable indicates the effect of an electrolyte containing only sulphuricacid for comparison purposes.

Current Anod- Film Temper- Voltdensity ising thickature, age (amps/ timeness Electrolyte range sqit.) (min.) (mil) Alloy 280 gin/litre H280 plus280 gmJlitre I-IgSOi 4 22-60 48 31 2. 4 H30 3 22-24 24 H15 230 gmJIitreHQSO; plus gm./litre NH NO 1 20-58 48 60 3. 2 H

280 gnL/litre H 804 plus 22.9 gm./lit1'e NH4N03 1 -90 48 41 2. 2 H30 120-70 48 60 3. 5 H15 280 gmJlitre H2504 plus 45.8 gmJlitre NH4NO3 020-54 48 60 3. 2 H15 280 gm./litre HzSOr plus 35.2 gmflitre urea nitrate2 24-00 48 33 2. 1 H30 2 18-50 48 39 3. 5 H15 280 gin/litre H2804 plusgnL/litre guanidine nit-rate 4 20-90 48 33 2. 2 H 4 23-51 48 46 3. 7 H15280 gmJlitre H280 plus 25 gmJlitre p-nitrophenol -1 20-50 48 60 3. 1 H153 24-90 48 28 1. 5 H30 280 gmJlitre H2804 plus 50 gm./litrem-nitrobenzene sulphonic acid (Na, salt) +3 27-35 48 60 3. 5 H15 +722-90 48 2. 3 H30 280 gm [litre H280; plus gm./litre m-nitrobenzoie 218-75 48 3.0 H30 acid (Na salt).

280 gm./litre I'IZSO4 plus 25 gum/litre Cu (N002 2 22-30 48 55 1. 4 H15280 gmJlitre H2804 plus 50 gm/litre C11(N03)2 0 23-37 8 54 4. 0 H15 015-42 48 25 2. 5 H30 280 gm./litre H280 plus gm. flitre C11(N0a)2 415-60 43 3 5 H30 With the use of sulphuric acid-nitrate ion electrolytesit has been tound possible to produce much thicker anodic coatings thancan normally be produced on certain aluminium alloys, notably thosecontaining appreciable amounts of copper or silicon, which arefrequently required to be hard anodised.

Moreover, the use of lower voltages made possible by the choice of thiselectrolyte considerably facilitates the continuous anodic oxidation ofaluminium wire and strip. In addition the present invention enablesanodic coatings of acceptable commercial thickness to be produced onaluminium alloys hitherto regarded as diflicult, such as thosecontaining more than 1% of copper, nickel or silicon or admixtures ofthese elements.

Although as described above it is assumed that the nitrate ion producingcompound is added to the sulphuric acid solution before anodisingcommences it will be understood that nitrate ion producing compoundcould be added to a sulphuric acid electrolyte during the anodisingoperation.

We claim:

1. A method of anodising aluminium and its alloy comprising the steps ofmaking up an electrolyte containing 140 gm. per litre of sulphuric acid,adding to the electrolyte before or during the anodising operation asoluble compound to produce nitrate ions in the electrolyte, the amountof such a compound being the equivalent of between 1 gm. and 45 gm. perlitre of nitric acid, and carrying out the anodising operation at atemperature between 0 C. and 55 C., at an applied voltage of between 10and 60, and at a current density of between 60 and amperes per squarefoot.

2. A method of anodising aluminium and its alloys in an electrolytecontaining a sulphuric acid solution comprising the step of adding tothe electrolyte before or during the anodising operation a solublecompound, to produce nitrate ions in the electrolyte, the amount of thecompound being such as to enable an anodised surface to be formed onaluminium or its alloy, wherein the amount of nitrate ion producingcompound is the equivalent of between 1 gm. and 45 gm. per litre ofnitric acid in the solution, wherein the nitrate ion producing compoundis selected from the group consisting of nitric acid, ammonium nitrate,cupric nitrate, urea nitrate, guanidine nitrate, m. nitrobenzoic acid,In. or p. nitrophenol and aniline nitrate, wherein the electrolyte is anaqueous solution comprising between 56 and 560 gm. per litre ofsulphuric acid, wherein the anodising operation is carried out at anapplied voltage between 10 and 60, and wherein the anodising operationis carried out at a current density between 60 and 120 amperes persquare foot.

3. A method according to claim 2, wherein the anodising operation iscarried out at an electrolyte temperature between 0 C. and 55 C.

References Cited UNITED STATES PATENTS 2,231,086 2/1941 Miiller 20458FOREIGN PATENTS 396,743 8/1933 Great Britain.

JOHN H. MACK, Primary Examiner R. L. ANDREWS, Assistant Examiner

